# Single Beam Acoustic Depth Measurement Techniques

## Quiz Questions

 1. Acoustic depth sounding was first used in the Corps back in the 1960s but did not replace reliance on lead line depth measurement until the 1970s or 1980s. True False 2. Acoustic depth measurement systems measure the elapsed time that an acoustic pulse takes to travel from a generating transducer to the waterway bottom and back. True False 3. Depth corrected to referenced water surface d = ½ (v • t) + k + dr ,what does k represent here? System index constant Measured elapsed time Average velocity of sound 4. Determining the sound velocity, v, is perhaps the most critical factor in using acoustic depth sounders. True False 5. The sound velocity does not vary with the density and elastic properties of the water. True False 6. The index constant does not contain any electrical and/or mechanical delays inherent in the measuring system, including return signal threshold detection variations. True False 7. The transducer draft and index constant must not be applied to the reduced time distance to obtain the corrected depth from the reference water surface. True False 8. The accuracy of the absolute time measurement generally varies with ______________. Distance Depth Velocity 9. __________________ are measured at the – 3 dB half-power points. Bandwidths Sensitivities 10. Higher frequency transducers have a frequency range of _______________________. 50 kHz to 800 kHz 100 kHz to 1000 kHz 100 kHz to 800 kHz 11. Lower frequency transducers tend to have __________________. Smaller beam widths Larger beam widths 12. The _______________ transducers provide more precise depth measurement. Lower frequency Higher frequency 13. The approximate bottom footprint size of a transducer can be computed by the following equation: Linear coverage (ft) = 2 • D • tan (a/2) Linear coverage (ft) =3.14 • D 2 • tan2 (a/2) 14. The wider the beam, the less effect vessel roll or pitch will have since the transducer beam width falls within the vertical. True False 15. The most commonly employed transducer frequency in USACE river and harbor navigation projects is______________________. 100-108k Hz 200-208 kHz 300-308 kHz 16. Figure9-4 represents ___________________________________. Typical single beam echo sounders used in Corps Raytheon DE 719 analog-recording portable echo sounder 17. The Hydrotrac is a single frequency, Recorder/Digitizer/Transceiver and is a highly integrated digital and analog sounder packaged into a small, waterproof housing. True False 18. _____________surveys are run either normal to (i.e., cross-sectioned) or longitudinal with the channel alignment. 50 and 200 ft 20 and 100 ft 19. _____________surveys are run either normal to (i.e., cross-sectioned) or longitudinal with the channel alignment. Single beam Double beam 20. ________________echo sounders typically collect depth data at a rate of 5 to 20 soundings per second. Single beam Double beam 21. An approximate computation of the update rate can be made from the following equation: Update rate (milliseconds) = 1185 • ( D / v ) • tan(a/2) where D is the average or project depth V is the velocity in knots a is the transducer beam width All of the above None of the above 22. Many districts have now incorporated motion compensation into single beam systems. Since vessel roll, pitch, yaw, and heave conditions can occur simultaneously and at different periods, either visual or automated interpretation of a single beam analog profile record to reduce these errors is an imprecise process, at best. True False 23. To best minimize the adverse effects of vessel motion, single beam systems used for dredging and navigation surveys in rough sea states should be equipped with automated heave sensors, and also pitch and roll sensors. True False 24. Excessive _________________ can inject position errors in the measured depth. Roll & pitch Pitch & heave Heave & roll 25. The important characteristic of pitch offset is that the along-track displacement caused by pitch offset is _____________________to water depth. Directly proportional Inversely proportional